‘86 XJ700 Maxim Speedometer and odometer issue

Hi.

I am experiencing a weird issue with the speedometer integrated with odometer of my XJ700. In a nutshell, the speedometer sometimes works, sometimes doesn’t. I do not find any regularity or dependency in it - it doesn’t seem to depend on the speed, weather conditions etc.

For context:
- I changed the speedometer cable 3 years ago to a brand new new one
- At that point I DID NOT put gear lube on the ribbed cable parts connecting it to the wheel rotation detector and the actual speedometer
- The issue started around 6 months ago
- I tried putting lube then, but the cable still sometimes works, sometimes doesn’t

Please, let me know what I can do to fix it and detect which of the parts is actually malfunctioning.

Thanks in advance!